One summer at Jackson Hole I noticed various helpful locals try to get a truck started owned by two just graduated college ladies. They ran a charger over to it, jumped it with jumpers, and measured it with a voltmeter. The battery was at full voltage. The headlights worked OK. When everybody finished shoulder shrugging, packed up and left, I asked the ladies what the story was. It was a new battery that somebody sold them about a week earlier and it worked fine usually, but not always. I grabbed the battery cable connectors and tried to twist them. I easily pulled one cable off. The ladies had some tools including a proper wrench. Truck started right up.
Connections are the electrical problem about 70% of the time.